🪟🎉 Connector builder: Improve testing panel UX on invalid configuration

[flash1293]

What

Fixes #20948 when working on that, I noticed a few other small issues that bothered me before, I tried to stabilize that as well.

Fixes a bunch of small inconsistencies in the testing panel:

• Field would not get touched on blur (airbyte-webapp/src/components/connectorBuilder/Builder/BuilderField.tsx) - add back the blur
• Stream selector didn't show new stream (instead renders undefined) if it got added in invalid state (airbyte-webapp/src/components/connectorBuilder/StreamTestingPanel/StreamSelector.tsx) - use the form state instead of the response from the listing call if the listing response is not available
• Instead of only rendering the error message on failing listing call, always show the stream selection and test button:
  • Keeps the UI stable
  • Use the UI state if the listing response is not available
  • Always keep the Testing button in view to jump to the first error

The last one is especially important while editing the yaml - in this situation it's expected to jump quickly between error state and valid state and this PR makes sure the UI is not jumping around:

Kapture.2023-01-31.at.14.22.54.mp4

This is how it looked before (multiple jumps):

Kapture.2023-01-31.at.14.24.43.mp4

How

• Expose stream list loading state and form values invalid state from the context so the testing panel can react to it
• Use the stream list from the form values if available and mix in names from the listing response
• Move the listing error message into the stream tester component so it's rendered right below the testing button

There is still a small delay before a newly created stream is shown in the testing panel - this is due to the debounce of moving the form values up to the builder state. I can't think of a good way to get rid of this at the moment, I hope that after the migration to react-hook-form we won't need this debounce anymore.

[lmossman]

Changes LGTM, tested locally and it works as expected. This UX is a lot better!